1. Field of the Invention
The field of the invention is imaging or display arrays having photosensor arrays having components embodying hydrogenated amorphous silicon (a-Si) technology, and more particularly, to a contact pad, as well a guard ring, having enhanced corrosion resistance while at the same time providing reliable electrical connections and also being particularly suited for use with an encapsulated data line having reduced electrical resistance. Such arrays may be used for X-ray or light imaging.
2. Discussion of the Prior Art
Imagers and display arrays have contact pads to which electrical contact can be made to external circuitry. Contact fingers connect the contact pads to the edge of the active array area where they electrically connect to scan or data lines or to the common electrode of the array.
The imager is formed on a substantially flat substrate, typically glass. The imager comprises an array of pixels with photosensitive elements, typically photodiodes, each of which has an associated switching element, preferably a thin film transistor (TFT). Both devices (photodiodes and TFTs) preferably comprise a-Si. In operation, the voltage on the scan lines, and hence that of the gates of TFTs of the pixels associated with each scan line, are switched on in turn, allowing the charge on each scanned line's photodiodes to be read out via the data address lines. The scan and data address lines are typically perpendicular to each other. The address line consists of a region in the array. The region outside the array comprises the contact finger, its associated contact pad and then, electrically insulated from the contact pad, a guard ring. The electrical contact to the guard ring is made via its own contact pads which do not electrically connect to the array. The guard ring is usually maintained at ground potential during operation. The guard ring serves the purpose of protecting the array from electrostatic discharge during formation of the imager, and during connection of the imager to external circuitry.
The contact pad is defined by an area of conducting material exposed on the substrate surface on a pad surface. The contact pad region, as used herein, includes the surface contact region and any additional regions with structures that electrically connect the surface pad to the main body of the contact finger. Usually the contact pad is at the end of the contact finger and the guard ring resides outside the contact pad. In some array embodiments, address lines may have two contact fingers and associated contact pads, at opposite sides of the array.
Contact pads consist of a single region TFT gate metal, gate dielectric with vias formed in them, source-drain (S-D) metal regions serving as electrodes, TFT passivation dielectric material typically comprised of silicon oxide (SiOx), a first layer of diode passivation material with a via formed through the two layers (TFT passivation dielectric and diode passivation materials), and a topmost conducting material typically comprising indium tin oxide (ITO) (which also usually forms a substantially transparent common electrode in the photodiode array). The imager includes other materials, such as TFT amorphous silicon (a-Si), photodiode a-Si, an overlaying thin ITO layer on the photodiode, and polymer dielectric, typically a preimidized polyimide (Pl) all of which materials are generally removed from the contact pad region. U.S. Pat. No. 5,233,181, assigned to the assignee herein, provides a description for a two layer diode passivation dielectric in which diode passivation layer formed of silicon nitride (SiNx) is removed from the contact pad during formation of the diode top contact via. It has been found that ITO is a good conducting material for use in imagers and display panels because it provides good electrical contact resistance and is particularly suited for use in a contact pad, but it is not a good barrier to moisture allowing possible corrosion of underlying metals.
It is thus desirable in a contact pad for an imager or display panel to use ITO as a conductor, but provide means to retard or even eliminate any corrosion of the contact pad from exposure to ambient moisture. It is further desirable that good electrical contact be maintained by conductive lines extending through vias disposed in passivation layers, such as thick inorganic dielectric materials disposed on the array.
Ground rings, in a manner similar to contact pads, suffer from corrosion when exposed to moisture which degrades the electrostatic protection and electrical function that the ground rings provide, and it is desirable to provide ground rings having means to retard or even eliminate corrosion of the ground rings when exposed to moisture.
The contact fingers, commonly employed in imagers and display arrays, electrically connect to the data lines of the active array. High performance imagers require low noise. Data lines suffer from having unwanted electrical resistance which increase Johnson-related noise during data readout, thereby degrading imager performance; it is thus desirable in an imager array to provide data lines with reduced resistance.
Solid state imaging devices are of particular importance to the present invention and typically include a photosensor coupled to a scintillator. Radiation absorbed in the scintillator (such as x-rays) generates optical photons which in turn pass into a photosensor, such as a photodiode, in which the optical photons are absorbed and an electrical signal corresponding to the incident optical photon flux is generated. The accumulated charge on the respective photosensors provides a measure of the intensity of the incident radiation. Such imaging devices commonly comprise an array of pixels arranged in rows and columns. Each pixel includes a photosensor that is coupled via a switching transistor (typically a TFT or the like) necessitating two separate address lines, a scan line and a data line, and a connection to a common electrode which electrically connects to one surface of all the photodiodes in parallel. In each row of pixels, the readout electrode of the transistor (e.g., the source electrode of the TFT) is coupled to a data line. The photosensor charge from each pixel is read by sequentially enabling rows of pixels (by applying an electrical signal to the contact pad and therefore to the TFT's respective gate electrode which causes the scan line to become conductive), and reading the photosensitive charge from the respective pixels thus enabled via respective data lines coupled to the TFTs.
Amorphous silicon is commonly used in the fabrication of photosensors due to the advantageous photoelectric characteristics of a-Si and the relative ease of fabricating such devices. In particular, photosensitive elements, such as photodiodes, can be formed in connection with necessary control or switching elements, such as TFTs in relatively large area arrays. Environmental conditions can affect the performance of the a-Si components; for example, performance is degraded by exposure to moisture in a manner similar to that discussed with reference to the contact pad and guard ring of the imager, which can be absorbed from humid air in the ambient environment. Moisture absorption in photodiodes undesirably increases the charge leakage from the diode.
Charge leakage is a critical factor in photodiode performance as the loss of charge during a sampling cycle lessens a photodiode's sensitivity and increases the noise. The two significant components of charge leakage are area leakage and sidewall leakage. Particularly in smaller diodes in which the length of the sidewalls is relatively large with respect to the overall area of the photodiode, sidewall leakage constitutes the primary source of leakage, although degradation of sidewall surfaces due to exposure to moisture can make sidewall leakage a significant leakage source in almost any size photodiode.
Multitier passivation layers are commonly made up of inorganic and organic dielectric materials as described in previously cited U.S. Pat. No. 5,233,181. The inorganic part of the diode passivation layer is typically comprised of silicon nitride while the organic passivation layer is commonly made up of polyimide. Most polyimides providing otherwise satisfactory passivation layer characteristics are hygroscopic, that is they tend to absorb some moisture from the environment. A dielectric material such as SiNx should have a high level of structural integrity to provide the desired moisture resistance and electrical insulation. This characteristic is particularly important as defects in the barrier layer disposed on the ITO common electrode can allow moisture penetration which in turn results in electrical leakage from the photodiodes: electrical leakage is an undesirable behavior that can seriously degrade imager performance by introducing electrical noise. The inorganic part of the diode passivation layer is disposed over steep sidewalls of the photosensor diode. Often, the points at which the inorganic part of the diode passivation layer is disposed are high stress areas in which structural degradation can result in moisture penetration and undesired electrical leakage through the diode passivation layer. Thus, structural degradation of the diode passivation layer creates higher electrical noise and a greater number of defective pixels in the imager array.
Although SiNx as the inorganic part of the diode passivation layer in sufficient thickness can provide an effective barrier to moisture penetration, use of SiNx can present processing problems. For example, a thick layer of SiNx is susceptible to cracking (both horizontally and vertically), thereby causing structural degradation and decreased resistance to moisture penetration. Poor adhesion may occur between SiNx and other layers, such as ITO which may be overlaying the photodiode surface or acting as a common electrode, or photoresist. The poor adhesion to photoresist can result in poor dimensional control in processing steps after deposition of the SiNx barrier layer, such as in the formation of vias to provide contact to the photodiodes.
It is thus desirable that an imager array demonstrate both a high degree of moisture resistance and structural robustness to enable effective fabrication and operation of the array in a variety of environments.